I remember back in the early and mid-90's the idea was to include micron-size vacuum tubes in integrated circuits in order to cut down the reverse-current to a level that semiconductor diodes could not do it.
Those tubes were functionally only diodes and the idea was to use the strong electric field at the tip of a very small cone to achieve cold electron emission. Imagine hollowing out a half-sphere and then add a cone with the tip at the center of the sphere. Now apply a voltage between the two. The electric field is very strong at the tip of the cone, but much weaker at the interior surface of the sphere. This results in cold electron emission from the tip of the cone and a current when the voltage is applied one way, but no current when the voltage is applied the other way.
I have never seen these used in practice. I believe one reason is that such a component had to be at least a couple of microns across, and the chips had little use for a 2000 nm diode.
Anyway, such vacuum tubes could have some use in parts where zero reverse current is important, but due to size limitation don't expect to see them replacing transistors that are counted in the billions in modern chips.